In a niobium titanium composite oxide such as Nb2TiO7, when Li is inserted, Nb is charge-compensated (from plus-pentavalent to plus-trivalent) and Ti is charge-compensated (from plus-quadrivalent to plus-trivalent). Thus, it has a high theoretical capacity of 387.6 mAh/g. Consequently, the niobium titanium composite oxide is expected to replace Li4Ti5O12 as a high capacity material; however, the rate performance is low. Generally, Nb-rich niobium titanium composite oxides, such as Nb10Ti2O29 and Nb24TiO62 are excellent in lithium absorption and release characteristics as compared with Nb2TiO7. However, the content ratio of Nb increases, which leads to an increase in cost.